The present invention relates to the thermo-optical recording of data which are to be optically read. It more particularly aims at a thermo-optical writing or inscription process using a low power laser and a thermosensitive support enabling on the one hand the reading of the data immediately following their recording without any intermediate development stage and on the other hand the copying of the data by an overall, rapid method without deterioration of the original etching or any modification of the etching characteristics during the copying process.
The structures proposed for writing data with low power lasers (typically below 15 mW) generally operate by thermal ablation. This is, for example, the case with layers of tellurium, bismuth, or vitreous chalcogenide alloy and even certain metal layers deposited on thermally degradable organic intermediate layers. The data are stored in the form of holes in said structures and during reading said holes create an amplitude contrast making it possible to detect the data. However, the data cannot be copied in an overall, simple and fast manner, for example by means of a polymerization process which forces a liquid to enter the cavity of the relief, followed by the hardening thereof by an appropriate means (thermal or photographic). This duplication method transforms the amplitude contrast of the original into phase contrast, i.e. the copies cannot be read with the same reader as that used for reading the original. A photographic or holographic method must be used to ensure that the copy retains an amplitude contrast. In this case the photographic method has serious disadvantages in that it requires an excellent contact between the emulsion and the recorded surface of the original, which is generally difficult to achieve over the entire area of the supports to be brought into contact due to dust. It is also necessary to work in a darkroom with top quality optical instruments, while using emulsions with a very high resolution. However, the holographic method requires cumbersome equipment.
Other structures have been developed by the Applicant having the special feature of providing better impression characteristics, particularly from the writing sensitivity standpoint and the reading signal to noise ratio than the structures operating by thermal ablation. These structures are constituted by a ductile metal layer covering a highly expansible organic layer. The light absorption by the metal layer during impact of the laser beam leads to the heating of the organic layer which reacts to this heating by a powerful local thermal expansion without change of state. In turn this expansion creates an unreversible stretching in the metal layer, exceeding the elastic limit, but remaining this side of the elongation at break. The main disadvantage of this type of structure is that the considerable thermal expansion of the organic layer is generally obtained by adding a large quantity of plasticizer to the polymer. However, the plasticizer is a liquid which behaves like a solvent and consequently leads to a drop in the mechanical properties of the polymer, particularly its hardness and its softening point. As a result particular care must be taken during the subsequent deposition of the metal layer to ensure that no mechanical stress is introduced into the organic layer which, on relaxing, may impair the morphology of the structure, both before and after storage of the data. Therefore the metallic layer must be deposited at a very low speed, be perfectly stainless, have a limited hardness, a limited mechanical rigidity and limited adhesion to the organic layer. As a result the data medium is difficult to manipulate without special precautions. In addition, the deposition of a random protective layer (thermosetting of photo-hardenable resin, varnish, etc.) is not possible, because it systematically leads to stresses which may denature the recorded impression. The only acceptable protection system is the use of a cap leaving a free space above the sensitive layer. However, this cap virtually doubles the cost of the data medium. Finally there is no possibility of using direct duplication by polymerizable liquid or contact photography for producing copies from such a fragile structure.